When hydrophobic particles are placed in water they tend to agglomerate. This tendency makes it difficult to obtain a dispersion of hydrophobic particles in a water-based system. To lessen this agglomeration tendency, a dispersant may be added to the mixture. If the dispersant is both hydrophobic and hydrophilic its hydrophobic part will be attracted to the hydrophobic particle surface and the hydrophilic part will be solvated by the water.
In recent years, the use of thermal ink jet printers in numerous applications has increased dramatically. Such printers use liquid-based inks, which are sprayed onto a receptor, typically a sheet of paper or film, to produce an image. By using basic ink colors (black, yellow, cyan, and magenta, or black, red, green and blue) as well as white pigments (such as TiO.sub.2) in various combinations and concentrations, virtually any color may be produced as part of the printed image. Additionally, ink jet technology is well-suited for high resolution graphic images, particularly those produced using electronic printing systems. Such systems typically employ computer technology to create, modify, and store images, text, graphics and the like.
Many of the inks that have been used in the past with ink jet and other printers are primarily comprised of dyes contained within organic-based carrier liquids. Although such inks may offer satisfactory performance in certain applications, the present trend is away from such systems, because such systems tend to produce images that lack the light stability and durability required for outdoor and similarly demanding applications. Additionally, the use of organic-based carrier liquids creates numerous environmental and material-handling complications. Rather, the printing industry has sought inks that are primarily water-based to thereby reduce or eliminate the problems associated with organic solvent-based systems.
Inks may comprise a suspension of pigment particles in a water-based carrier. However, the suspended pigments may tend to agglomerate. Because ink jet printers use very small jet nozzles (on the order of less than about 40 micrometers with droplet volumes on the order of picoliters) to provide high resolution images, pigment agglomerations may restrict or clog the printer heads. This effect is referred to herein as "plugging." Additionally, in the case of thermal inkjet systems, the ink is subjected to high temperatures (approximately 350.degree. C.) as a result of the heating element in the nozzle. Typically inks tend to agglomerate at a faster rate at higher temperatures, and may also settle onto, and coat, the heating elements of the printer heads. This causes decreased thermal efficiency of the print head which results in the formation of smaller ink droplets and lower image quality. This effect is commonly referred to as "kogation."
To overcome the problems described above, some pigment particles have been stabilized (from agglomeration) with dispersants. In one approach, the dispersants were formed from surfactants having a hydrophilic portion and a hydrophobic portion, which hydrophobic segment adsorbed onto pigment surfaces of varying hydrophobicity. In another approach, copolymers having hydrophilic and hydrophobic segments were used. Examples of these approaches are described in U.S. Pat. No. 4,597,794 and U.S. Pat. No. 5,085,698.
In the approaches described above, the hydrophobic segments of the surfactant or polymer can adsorb onto pigments by hydrophobic interaction between the dispersant molecule and the organic-based pigments whose surfaces tend to be hydrophobic. This hydrophobic interaction is usually not very strong. Because of this weak attraction, the dispersant molecules may desorb from the pigment surface, thereby allowing the pigment particles to agglomerate. During printing, this can result in plugging of the printer head jet nozzles. Although heat produced in a thermal inkjet system may strengthen the adsorption of the dispersant on the hydrophobic pigment surface, thus enhancing the stability of the ink, plugging and kogation remain a problem.